Semiconductor devices are made by forming a plurality of features in or on a surface of a substrate using photolithographic techniques. Each feature, when formed, must be aligned with previously formed features. To achieve this alignment, it is the practice to provide an alignment key on the surface of the substrate and corresponding alignment keys on each of the photolithographic masks. When the key on the mask is aligned with the key on the substrate, the features to be formed by the particular mask will be aligned with previously formed features. The alignment of the mask key with the substrate can be achieved either manually by looking at the mask on the substrate through a microscope or with a television camera, or automatically, such as by an apparatus shown in U.S. Pat. No. 4,422,763 to H. P. Kleinknecht, entitled "Automatic Photomask Alignment System For Projecting Printing", issued Dec. 27, 1983.
Semiconductor devices have been developed which include features on both sides of the substrate with the features on one side being aligned with the features on the other side. One technique which has been used to achieve alignment of features on both sides of a substrate has been an alignment apparatus which uses infrared (IR) radiation which passes through the substrate and illuminates both sides of the substrate. However, this has disadvantages when the substrate contains features, such as metal coatings, which are not transparent to IR radiation. Also, this type of apparatus is not easily made compatible with high-resolution, low-defect density projection lithography and suffers from alignment errors resulting from loss of resolution in the IR optical system. Another technique is apparatus which has appropriate optics to see both sides of the substrate simultaneously. This type of apparatus has the disadvantage of being complex and expensive.